Other answerer had good advice about honda or toyota, I will also add nissan and subaru. 4 grand is a good price range for a good used mustang. The best thing I can tell you is be choosy,and have lots of patience and do not buy some kids whipped old high mileage mustang. A private party add out of a big city newspaper is best, most old people with nice cars have no idea who a craigslist is and could care less about the internet. And do not be lookin on a dealer lot, you end up with a high mileag problem in that price range. 4 grand can work good, it takes patience and time on your part. I know you dont have any but that is what your best chance is of makeing the money work for you. Also , some of the nicest cars I ever found were on the side of the road for sale. So you never know. I like to do a cold start test drive on any used car.
I ask the owner not to start it the night before , and show up in the morning check the engine oil to see if its clean (and the same temprature as the outside air) check the tranny fluid on an automatic to make sure its pink and clear (good) not black or burned (bad, walk away) If the tranny fluid is good I start up the engine and listen to it cold. It should start easily , no loud bangs or clouds of blue smoke out the tailpipe. And should smooth into a high idle in 5 to 10 seconds. I give it a minute or two for oil circulation and then jump in it and drive it like I stole it. Any used car that you can get to move and get up to freeway speeds 55 miles per hour in the first 5 to 7 minutes when cold , may not be in too bad of shape. I like to test drive a car for 40 minutes to an hour. I check all the accessories and make sure everything works. ( I have already discussed the long test drive with the owner) If an owner does not want to let you test drive a car, or says no to the next request (which is taking it to a mechanic) dont even waste your time looking at it.
A mechanic is your last line of defense against a crap car, the one thing most people skip, and the one thing you can not afford to skip. People will lie and cheat, to sell a car with problems. If your extended test drive finds none, and your mechanic finds no glaring defects in engine or transmission, its likely a pretty good car. I also do not go look at unregistered cars. Some owners are so cheap, that when they know they are selling a car they refuse to pay tags. If you cannot test drive it then its worthless. And im not test driving a unregistered uninsured car, the cops will impound it any way. And what other maintenance was skipped? no deal
here is a website that may help www.samarins.com
has lots of information on buying a used car. another one is wwwcarcomplaints.com not every car with one complaint is a lemon, but for example some american cars with thousands of complaints over severl models and years, dont buy one. honda has some bad years with accords and civics, the good news by now, most of the crap is gone. patience , patience and more patience, is the best way to buy a used car. And www.nada.com
has a consumer price guide section that can give you an idea of pricing information for many year makes and models. good luck